The invention generally relates to an optical display device, and more particularly, the invention relates to a display panel, such as an active matrix liquid crystal display (LCD) panel, for example.
Referring to FIG. 1, an active matrix liquid crystal display (LCD) panel 1 may include an array 6 of pixel cells 25 (arranged in rows and columns) that form corresponding pixels of an image. To accomplish this, each pixel cell 25 typically receives an electrical voltage that controls optical properties of the pixel cell 25 and thus, controls the perceived intensity of the corresponding pixel. If the pixel cell 25 is a reflective pixel cell, the level of the voltage controls the amount of light that is reflected by the pixel cell 25.
There are many applications that may use the display panel 1. For example, a color projection display system may use three of the display panels 1 to modulate red, green and blue light beams to produce a projected multicolor composite image. As another example, a display screen for a laptop computer may include a display panel 1 along with red, green and blue color filters that are selectively mounted over the pixel cells to produce a multi-color image.
Regardless of the use of display panel 1, updates are continually made to the voltages of the pixel cells 25 to refresh or update the displayed image. More particularly, each pixel cell 25 may be part of a different display element 20 (a display element 20a, for example), a circuit that stores a charge that indicates the voltage for the pixel cell. The charges that are stored by the display elements 20 typically are updated (via row 4 and column 3 decoders) in a procedure called a raster scan. The raster scan is sequential in nature, a designation that implies the display elements 20 are updated in a particular order such as from left-to-right or from right-to-left.
As an example, a particular raster scan may include a left-to-right and top-to-bottom "zig-zag" scan of the array 8. More particularly, the display elements 20 may be updated one at a time, beginning with the display element 20a that is located closest to the upper left comer of the array 6 (assuming the display panel 1 is standing upright). During the raster scan, the display elements 20 are individually and sequentially selected (for charge storage) in a left-to-right direction across each row, and the updated charge is stored in each display element 20 when the display element 20 is selected. After each row is scanned, the raster scan advances to the leftmost display element 20 in the next row immediately below the previously scanned row.
During the raster scan, the selection of a particular display element 20 may include activating a particular row line 14 and a particular column line 16, as the rows of the display elements 20 are associated with row lines 14 (row line 14a, as an example), and the columns of the display elements 20 are associated with column lines 16 (column line 16a, as an example). Thus, each selected row line 14 and column line 16 pair uniquely addresses, or selects, a display element 20 for purposes of transferring a charge (in the form of a voltage) from a video signal input line 12 to a capacitor 24 (that stores the charge) of the selected display element 20.
As an example, for the display element 20a that is located at pixel position (0,0) (in Cartesian coordinates), a voltage may be applied to the video signal input line 12 (at the appropriate time) that indicates a new charge that is to be stored in the display element 20a. To transfer this voltage to the display element 20a, the row decoder 4 may assert (drive high, for example) a row select signal (called ROW.sub.0) on a row line 14a that is associated with the display element 20a, and the column decoder 3 may assert a column select signal (called COL.sub.0) on column line 16a that is also associated with the display element 20a. In this manner, the assertion of the ROW.sub.0 signal may cause a transistor 22 (of the display element 20a) to couple a capacitor 24 (of the display element 20a) to the column line 16a. The assertion of the COL.sub.0 signal may cause a transistor 18 to couple the video signal input line 12 to the column line 16a. As a result of these connections, the charge that is indicated by the voltage of the video signal input line 12 is transferred to the capacitor 24 of the display element 20a. The other display elements 20 may be selected for charge updates in a similar manner.
Referring also to FIGS. 2, 3, 4 and 5, a row of the display elements 20 may be scanned in the following manner. First, the row decoder 4 continuously asserts (drives high, for example) the ROW.sub.X signal (the ROW.sub.0, ROW.sub.1, . . . or ROW.sub.N signal, as examples) that is associated with the particular row. While the ROW.sub.X signal remains asserted, the display elements 20 of the selected row are sequentially selected in column order to receive a time slice of the voltage of the video signal input line 12. In this manner, the column decoder 3 individually and sequentially asserts (drives high, for example) the COL column select signals (the COL.sub.O, COL.sub.1, . . . and COL.sub.M signals, as examples), as depicted in FIGS. 4 and 5 for the COL.sub.0 and COL.sub.M signals, while the row decoder 4 keeps the ROW.sub.X signal asserted. The selection of each display element 20 (and the associated charge transfer) may consume a cycle of a clock signal (called CLK and shown in FIG. 2). Thus, a scan of M (i.e., the number of columns) display elements 20 of a row may consume approximately M clock cycles.
In the above-described approach, all of the display elements 20 are sequentially and individually selected according to a predefined raster scan sequence, a technique that may limit the rate at which a particular portion of the array 6 may be updated. For example, the display 1 may be used to display picture frames of a video image, and between two successive frames, some portions of the image may change more rapidly than other parts of the image. Unfortunately, the maximum rate at which the more rapidly changing portions may be updated may be limited by the rate at which the raster scan is performed. As a result, temporal artifacts, or errors, in the video image may be more apparent in the more rapidly changing portions of the image.
Thus, there is a continuing need for an arrangement that addresses one or more of the above-stated problems.